coelostat


A flat mirror fitted in a mount that is mechanically driven so that, as the Earth rotates on its axi s, light
from a chosen area of the sky can be continuously directed into an optical analysing instrument. Whereas
a telescope is rotated by its mounting to keep the image within its field of view, this is not always
practical for heavy optical analysing equipment. Therefore it is often simpler to keep the equipment fixed
and to use a coelostat to follow the rotation of the celestial sphere, constantly guiding light into the main
optical system. The heliostat performs a similar function for keeping an image of the Sun fixed.